Selective Catalytic Reduction (SCR) of oxides of nitrogen (NOx) by nitrogen compounds, such as ammonia or urea, has proven to be effective in industrial stationary engine applications. More recently, ammonia-SCR has been incorporated into mobile diesel internal combustion engines such as heavy-duty truck and bus engines.
Generally, in the ammonia-SCR process, an aqueous water solution of urea is injected into the exhaust gas stream of an engine. At temperatures above 160° C., the urea begins to undergo hydrolysis and thermal decomposition resulting in the production of ammonia. The resulting mixture including urea/ammonia and exhaust gases then passes to an SCR catalyst such as platinum (Pt), vanadium (V2O5), or zeolite, where the ammonia reacts with NOx gases to form nitrogen gas and water.